evas/cserve2: fix crash when shutting down cserve2

If a slave is still alive during shutdown, then it will be killed
and the dead callback should be called. This would then trigger
a request failed callback on invalid data.
But there's no point in calling the dead_cb since we're shutting
down already.

src/bin/evas/evas_cserve2_slaves.c

@@ -302,6 +302,9 @@ cserve2_slaves_shutdown(void)
 
    EINA_LIST_FREE(slave_procs, sp)
      {
+        Slave *s = (Slave*) sp;
+
+        s->dead_cb = NULL;
         kill(sp->pid, SIGKILL);
         _slave_proc_free(sp);
      }